The present invention relates to coated abrasive discs and to an economical method of making coated abrasive discs adapted for easy modification to meet specific requirements.
Traditionally abrasive discs comprise a substrate which may be made of polymer film, paper, or a knit, woven or stitch-bonded fabric. The backing may need to be xe2x80x9cfilledxe2x80x9d to ensure that a binder applied thereto does not become absorbed into the material. This may be referred to as a xe2x80x9csizexe2x80x9d and may be applied to the front, back or both sides. A binder, called the xe2x80x9cmakexe2x80x9d coat, is applied to the backing and before the binder is cured, abrasive grits are applied to the binder and the binder is then cured to anchor the grits in place. A second binder layer also, (perhaps confusingly), called a xe2x80x9csizexe2x80x9d coat, is usually applied over the grits to complete the anchoring of the grits.
In conventional manufacturing the above process is applied to a continuous sheet and the individual discs are punched out from a large roll of the sheet, called a xe2x80x9cjumboxe2x80x9d. Even with the closest possible spacing of the punched out shapes, there is a significant amount of waste in terms of backing, abrasive grain applied and binder used to anchor the grain. The larger the disc diameter, the larger the amount of waste. In addition the production method requires that the disc have a uniform construction at all points since the same jumbo may be used to produce discs of various diameters and even belts.
However, the way an abrasive disc is conventionally used, only the outside edge of the disc is actually used before the disc is considered worn out because of the angle at which the disc is presented to the workpiece. Thus the usual methods of making the discs is wasteful as made from a jumbo and as used in practice.
The present invention provides a means of making abrasive discs more economically and this leads to the possibility of making novel abrasive disc structures that can be designed to provide significant advantages over the prior art.
The whole concept of the design of a coated abrasive disc is changed when it is appreciated that abrasive discs can be individually made rather than cut from a larger jumbo roll and the present invention was stimulated by the realization by the inventor that a technique could be devised by which abrasive disc could be individually produced and specifically designed for the intended application. The present invention therefore provides an abrasive disc having first and second major surfaces, said first surface having a primary abrasive area which covers only the outer peripheral portion of the first surface and extends from the periphery to a point that is at least 5% and up to 50% of the radial distance to the center of the disc. The primary abrasive area of the disc is preferably provided with a premium abrasive-containing abrasive layer. The balance of the surface of the disc, (the central area), can be devoid of abrasive or possibly covered by less abrasive or by a different, perhaps more friable, abrasive or an abrasive mixture in which a lower quality abrasive predominates. Alternatively and often preferably the remaining portion of the abrasive area can be provided with a pattern of abrasive in isolated structures on the surface. Very often the transition from primary abrasive area to the central area is not abrupt but more gradual with some degree of overlap between an area bearing a higher quality abrasive and one bearing a lesser quality abrasive, thereby masking the transition.
The central area need not be uniform and indeed it is often desirable to define two or more portions within the central area. Thus the central area can comprise one or more outer annular sections and an axial section. Outer annular sections can form a transition between the primary abrasive area and the axial section which can be devoid of abrasive. The outer annular sections can comprise progressively less abrasive, (even the premium abrasive used in the primary abrasive surface), with distance from the periphery, or the abrasive can be a mixture of inferior with the superior abrasive with the inferior proportion increasing with distance from the periphery. Generally, though not essentially, the axial or innermost, section is left devoid of abrasive altogether since it never contacts a workpiece. It can however be covered by a lower quality abrasive if desired.
The abrasive material in the primary abrasive area is typically fused or sintered alumina, silicon carbide or fused alumina/zirconia. It is however preferably a premium, in the sense of being more effective for the desired application, abrasive However it is to be understood that the xe2x80x9cpremiumxe2x80x9d quality can also derive only from the comparison with the amount and quality of the abrasive (if any) in the central area of the disc. Thus where there is no abrasive as such in the axial section of the disc, the most common fused aluminum oxide can become the xe2x80x9cpremiumxe2x80x9d abrasive. By the same token if the abrasive in the peripheral primary abrasive area is a filamentary sintered sol-gel alumina abrasive, fused alumina could certainly be incorporated in some or all of the central area of the disc as an xe2x80x9clower qualityxe2x80x9d abrasive. More generally however, where the central area of the disc has a coating comprising a lower quality abrasive material this can even be sand, a crushed mineral such as limestone, ground glass, particulate ash or clinker and the like.
The abrasive can be bonded to a substrate using a maker layer or the abrasive can be dispersed in a curable bond material which is applied to a backing material and subsequently cured. The latter technique is more often used with finer grade abrasive materials used primarily for developing surfaces with fine finishes. The most useful field for the application of the present invention is in the production of abrasive discs in which a disc backing material first receives a maker coat of a curable resin formulation and the abrasive is applied to the backing material either by a gravity feed or by electrostatic projection and the maker is then at least partially cured before a size coat of a resin compatible with the resin providing the maker coat is deposited over the abrasive grains. Cure is typically then finished for maker and size coats simultaneously. A supersize coat comprising a surface properties modifying additive, (such as a lubricant, anti-static additive or a grinding adjuvant), dispersed in a curable binder resin can be applied over the size coat if desired.
The backing material upon which the abrasive material is deposited can fibrous, paper or film. Fibrous backing materials are most frequently encountered in the applications for which the present invention is primarily useful though there is nothing inherent in the invention that so limits its scope. The fibrous backings may be based on woven fabrics, non-woven materials such as stitch-bonded fabrics, needled felts, or knit fabrics. Such a fibrous backing material is typically pre-sized with a filler in a back-size or front-size so as to fill up the pores of the fabric before the maker coat is applied such that the maker coat remains essentially on the surface. In some cases the fibers are completely or almost completely embedded in a thermoplastic or thermosetting resin matrix in which case pre-sizing of the substrate is not required.
The present invention also comprises a process for the manufacture of abrasive discs having a peripheral primary abrasive area extending from 5 to 50% and preferably from 10 to 50% of the distance from the periphery of the disc to the center which comprises feeding an abrasive grain to a grain deposition surface over the outer surface of a cone such that the deposition surface receives an annular deposition of the grain. The outer annular portion around the rim can have a thickness, (in the radial direction), towards the narrower end of the above range when at least part of the inner portion is provided with a coating of abrasive in the form of isolated islands of abrasive which are particularly advantageous when the abrasive disc is used in an environment that generates a troublesome amount of swarf that could shorten the useful life of the disc. The grain deposition surface can be the primary abrasive area itself where the disc comprises a backing material that has been coated with a maker coat and if the deposition of the grain is by a gravity technique. More often however it is a surface, such as a moving belt surface, from which the grain will deposited by a UP technique on to a disc of a backing material that has been coated with a maker coat. The deposition surface is preferably provided with a circular peripheral wall defining the area from which the grain will be projected during the UP deposition process. This helps to concentrate the grain on a specific area of the grain deposition surface and avoids any losses to the surroundings.
Where it is desired to provide annular rings comprising different abrasive grains within the central area of the abrasive disc, this can readily be accomplished by providing a series of cones with different greatest diameters but a common axis accommodated within the cone over which the abrasive grain is distributed for deposition on the primary abrasive area. In each case the grain is preferably distributed over the surface of the cone through distribution channels feeding only that specific surface. Uniformity of distribution within the distribution channels can be promoted by interposing one or more horizontal screens between the point at which the grain enters the distribution channel and the point at which it is discharged on to the distribution surface. Such screens are preferably agitated while grain is passing through the screens to promote uniform distribution within the channel.
Islands of abrasive in the inner portion of the abrasive area can be produced by application to the backing through a mask with appropriate apertures or it can be produced by applying a maker coat with an appropriate pattern and then applying abrasive grain over the backing such that adhesion only occurs in the area treated with the maker coat.